In a typical rear wheel drive powertrain, an internal combustion engine converts chemical energy into mechanical energy to rotate a shaft and a transmission adapts the speed and torque of the shaft to suit vehicle requirements. At slow vehicle speed, the transmission reduces the speed and multiplies the torque to improve acceleration. At cruising speeds, the transmission increases the speed allowing the engine to operate at a fuel efficient operating speed. Power is transferred from the transmission output to the vehicle wheels via a rear driveshaft, a rear differential, and rear axle shafts. The transmission may be an automatic transmission which establishes one of a fixed number of available power flow paths by engaging particular friction clutches. The clutches may be engaged by supplying pressurized fluid through a hydraulic control system.
To improve traction, it is advantageous to be able to transmit power to all four vehicle wheels. To accomplish this, a transfer case mounted to the transmission may distribute power from the transmission output to the rear driveshaft and also to a front driveshaft that drives the front wheels via a front differential and front axle shafts. Many transfer cases include a torque on demand (TOD) clutch that selectively transfers power to the front driveshaft. Typically, control of the TOD clutch is independent of the transmission clutches.
Many transfer cases also include a low range and a high range to provide added vehicle functionality. Control of the coupler that selects the desired range is also typically independent of the transmission clutches. Some transfer cases are also capable of selecting a neutral position in which the front and rear driveshafts are not coupled to the transmission output. This is useful for towing the vehicle because movement of the vehicle results in rotation of the front and rear driveshafts. However, since some transfer case components still rotate, proper lubrication of those components is still required during towing.